1. Field of the Invention
The invention relates to communication systems, and in particular to electronic devices and methods for improvement of radio transmitter and receiver frequency accuracy of a local radio communication unit that communicates with a remote communication unit.
2. Description of the Related Art
Communication systems often comprise a plurality of local units such as radiotelephone handsets that communicate digital data by radio transmissions with a remote unit such as a cellular phone base station. The radio frequencies of the communication channels and frequency error tolerances for transmissions on the channels are typically specified by regulatory rules. The frequency tolerances ensure that the level of radio interference between channels is tolerable and that accurate data demodulation is possible at the local unit and the remote unit. In the base stations, the transmitter and receiver radio frequencies are typically phase locked to very stable reference oscillator signals available, in order to meet regulated radio frequency tolerances. However, the cost of the stable reference oscillators is typically very high for radiotelephone handsets. As such, provision for accurate transmitter and receiver frequencies in the local unit at the lowest possible cost is important.
For local communication units, the conventional solution for accurate radio frequencies is to use a relatively low cost voltage controlled crystal oscillator (VCXO) to serve as a reference oscillator, wherein the oscillator frequency is approximately linearly related to the magnitude of a VCXO control voltage. Sometimes, the VCXO control voltage is adjusted based on estimated radio frequency error of the receiver in accordance with well known feedback control principals, such that the radio frequency errors of the receiver and transmitter are sufficiently reduced by feedback control principals. The methodology for radio frequency control in the local unit is a conventional automatic frequency control (AFC) loop.